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netrable till they shall be studied on principles analogous to those which have guided the author?
Nevertheless, these researches have already thrown much light on many important but obscure questions; and independently of the interest which, in a purely chemical view, they must always have for the chemist, they will be found, by the physiologist and the medical man, both interesting and valuable in a very high degree.
In connection with previous researches, they serve to demonstrate, that the more we know of the processes going on in the organism, the more do we find these to involve strictly chemical changes, and to be capable of a chemical interpretation. It would indeed appear as if every change in the organism were attended by a' definite chemical or physical action; and although we shall probably never succeed in unveiling the nature of the peculiar influence, called vitality, under which these changes occur, yet the present as well as previous investigations render it certain that we have still a great deal more to discover concerning the share taken by chemical action in the vital processes.
I cannot omit to direct the attention of physiologists to the proofs, contained in the following pages, of the truth of the principle, that every property, however apparently trifling or minute,
possessed by any constituent of the organism, even by such as occur only in very small proportion, has its destined use and function; and, consequently that every constant difference, whether of composition, of form, or of quality, in the different tissues and fluids, must likewise correspond to a difference of function, in which, as a general rule, it cannot be replaced, nor its absence compensated for, by any other substance, however analogous in most of its properties.
A striking example of this truth will be found in the facts concerning the great preponderance of phosphate of potash and chloride of potassium in the juice of flesh, while in the blood and lymph which circulate through the muscles, it is phosphate of soda and chloride of sodium which prevail. Another will be found in the fact that the juice of fesh is always strongly acid, while the blood and lymph are decidedly alkaline; and a third is seen in the abundant supply of lactic acid in the juice of flesh, while it cannot be detected in the urine.
But perhaps the most interesting observation, next to the discovery of kreatine as a constant ingredient of flesh, of kreatinine, a powerful base, in the juice of flesh, and of both in urine, is the demonstration, complete, as it appears to me, of the true function of the phosphate of soda in the blood.
This function, that of absorbing carbonic acid and giving it out in the lungs, is here shown to depend entirely on the minute chemical characters of the salt in question ; and we now see how it happens that phosphate of soda is essential to the blood, and cannot be replaced by phosphate of potash, a salt, which, although in many points analogous, differs entirely from phosphate of soda, in its tendency to acquire an acid instead of an alkaline reaction, and in its relation to carbonic acid. In this way, the beautiful researches of Graham on the phosphates are now finding their application, in the minutest point, to physiology. The same remark applies to the action of common salt on phosphate of potash, which satisfactorily accounts for the presence of phosphate of soda in the blood of animals whose food contains only phosphate of potash, but which either find common salt in their food, or obtain it as an addition. Surely such facts as these must convince all men of the value of the most minute study of the chemical properties of all the substances which occur in the organism, however these properties may at first appear trifling or unimportant; and of the utter impossibility of making progress in Physiology without the aid of Chemistry. I would also direct attention to the evidence here given of the fact, that the parietes of
the different systems of vessels, as well as the membranes and cells, must possess, in the living body, a power of selection, or, in other words, different degrees of permeability, in reference to the various substances which penetrate them by endosmose. To this subject the investigations of the Author have been more particularly directed, since the termination of the present work; and results of great interest and value have been already obtained.
The medical man will find in these Researches a prospect of many and great improvements in practice, whether as regards dietetics, or the action of acids, alkalies, and salts on the digestive and respiratory processes ; and with respect to both, it is to Chemistry that he must look for assistance in his efforts to advance. Lastly, the present work contains some most valuable practical applications of the chemical discoveries therein detailed, to an art which immediately concerns the whole of mankind ; namely, the culinary art.
The subjects of the preparation of meat for food by boiling, roasting, and stewing; the true nature and proper mode of preparation of soup, as well as of the extract of flesh or genuine portable soup; and, finally, the changes produced in meat, not only by the above processes, but by salting, and the conditions necessary in each case to insure
the digestibility and nutritive qualities of the Aesh or soup, are here, for the first time, investigated on scientific principles; and in all these points, Chemistry is found to be the means of throwing light on that which was obscure, and of improving our practice by the application of rational principles.
In conclusion I would remark, that the apparent simplicity of the results, and even of the processes described, gives a very inadequate idea of the laborious and difficult nature of the investigation. Having myself repeated several of these processes, I have been enabled to perceive, that, unless Baron Liebig had devoted to the subject his whole energies for a long time, and unless, moreover, he had operated on a scale so large as few experimenters would have ventured on, the whole subject would have remained as obscure as ever. Not the least valuable lesson to be derived from this work is the absolute necessity of experimenting on a very large scale, if we would obtain satisfactory or trustworthy results.
UNIVERSITY OF EDINBURGH,
31st May, 1847.